This page is an essay on the deletion policy.
No Encyclopedic Use is a common criteria for the deletion of files that is often evoked when the subject of the picture is someone or something non-notable, or else a small piece of something notable, that due to its cropped size or other issues aren't usable.
Rarely is a picture notable in and of itself, aside from some famous historical pictures, it is usually the subject that is notable. One common rule-of-thumb one can use is to ask, "Could this subject (of the picture) have an article made about them and not be deleted?"; if the answer is no, then it likely isn't notable, unless of course it serves as a "support image". Support images are those that aren't directly related to the subject, such as an article about Cotton might have a picture of an inventor who revolutionized cotton picking.
Sometimes however, images of notable things meet the "No encyclopedic use" criteria. This may be due to it being cropped in such a way that the subject is unidentifiable, for example an image of Jimbo cropped so that only his shoulder is visible, or else it may be unusable due to it having something copyrighted in it; by not meeting the De minimis standard, it cannot be used.